In printers such as inkjet printers having traveling inking means (e.g., inkjet pens), ink drops follow trajectories determined by the vector sum of the ink ejection velocity (Ve) and the velocity of the inking means (Vp). For example, in an inkjet printer providing resolution of about 300 dots per inch, a typical pen velocity would be about 0.34 m/sec and a typical inkjet ejection velocity would be about 5 m/s. If distance Dp is defined as the distance measured laterally along the surface of a printed sheet between the inking means and the intended location of ink dot placement on a sheet at the time of inkdrop ejection, and if distance Ds is defined as the pen-to-sheet spacing as measured perpendicular to the sheet surface, then the ratio of Dp to Ds is proportional to the ratio of the velocity Vp to the velocity Ve. Thus, assuming that the controllable variables Vp and Ve are fixed for a particular inkjet printer, the lateral distance Dp can be calculated to equal the quantity ##EQU1## Ideally, distance Dp remains constant whenever a sheet is being printed to avoid misalignment of printed characters; however, because pen-to-sheet distance Dp is a function of distance Ds, the latter distance must also remain constant to maintain accurate ink drop placement during printing.
The maintenance of constant pen-to-sheet spacing distance, Ds, is especially critical in inkjet printers of the bidirectional type. In such devices, an inkjet pen prints a swath of ink drops while moving both from right-to-left and from left-to-right across the surface of a sheet. Normally, between each change in printing direction in bidirectional inkjet printers, the printed sheet is indexed a swath width (e.g., about 3/8 inch). Because such printers provide ink dots in columns in each swath, print defects will appear unless dot columns on adjacent swaths are closely aligned. In fact, it has been calculated that print defects will be perceived unless dot columns on adjacent swaths are aligned to within 1/10 of a dot diameter, or about 0.00033 inch at a resolution of about three hundred dots per inch. At the velocities described in this example, such alignment of dot columns in successive swaths requires that the pen-to-sheet spacing distance Ds be held to tolerances of about .+-.0.0025 inch.
Because of the precise tolerances required, conventional inkjet printers are often unable to provide consistently acceptable print quality. In fact, in conventional inkjet printers, the additive effect of manufacturing tolerances often cause pen-to-sheet spacing distance Ds to vary substantially more than desired. Also, the spacing distance Ds in conventional inkjet printers can be affected by lack of flatness in carriage guides and paper support plates.
Further, ink dot placement during printing can vary because of variations in sheet thickness and because of curls and cockles in sheets. For example, sheet thicknesses commonly used in printers vary by about 0.002 to about 0.007 inches. Also, cockles can be present because of paper defects and because of moisture present during printing.
To reduce the effects of paper curl and cockle on dot placement during printing, conventional practice is to employ sheet holddown devices such as electrostatic or suction devices. In an electrostatic holddown device, for example, paper flatness is maintained by establishing electrostatic attraction between a flat support plate on the printer and the back surface of a sheet to be printed. Likewise, in vacuum holddown devices, sheet flatness is maintained by providing suction between a support plate and the back surface of a sheet to be printed. It should be noted that, in either type of holddown device, direct contact of the holddown device with the printed surface is avoided to minimize ink smearing and other adverse affects on print appearance.
Although conventional holddown devices are fairly effective in maintaining sheet flatness during printing, they have drawbacks. One drawback is that such devices do not compensate for variations in sheet thickness. Another drawback is that the maximum holddown force on a sheet is limited because of the necessity to maintain low frictional loads on transport devices which index the sheets. In conventional inkjet printers, such limitations can cause pen-to-sheet spacing distances to vary from swath to swath. Also, the holddown pressure at a localized area being printed may be insufficient to flatten cockles and other paper irregularities; that is, the pressure required to flatten cockles in a sheet may be too great to allow precise paper indexing, especially in vacuum devices which exert pressure over the entire surface area of a sheet. Finally, conventional holddown devices are complicated and relatively expensive.